A. Field of Invention
The present invention relates generally to pressure relief valves and more particularly to valves used to control pressure in hydraulic or pneumatic systems.
B. Description of Related Art
Pressure relief valves are available which employ a ball or a poppet biased against a valve seat in communication with a fluid medium. When the pressure exerted by the fluid medium on the ball or poppet exceeds the force exerted by the biasing means, the ball or poppet is disengaged from its seal with the valve seat which allows flow through the valve, thereby relieving and reducing the pressure within the system. When the pressure of the fluid medium in the system is reduced to less than the force exerted by the biasing means, the ball or poppet returns to a sealing engagement with the valve seat. In the conventional pressure relief valves of this type compromises must be made in at least one of the following variables: size, flow, leakage and the valve response characteristics or hysteresis. Commonly, in miniature valves for use in high pressure applications, acceptable rates of flow through the valve are achieved at the cost of unacceptable leakage rates and relatively high hysteresis. The typical ball or poppet and the valve seat with which it engages must be nearly one hundred percent concentric to attain proper sealing characterized by highly uniform seat stresses. Such exact specifications are extremely difficult to achieve, particularly in valves manufactured for miniature applications.
In conventional relief valve designs the valve seat is an integral part of the valve and subject to distortion by installation stresses. It is commonly necessary to subject the valve to axial or radial compression when sealing the orifice or bore into which the valve is typically inserted. Under such external loading the requisite valve seat-poppet concentricity is often lost.
The use of soft materials in the elements of the seal may provide a low leakage rate but generally results in a valve that lacks sufficient integrity for use in high pressure applications. Increasing the area of the interface between the valve seat and the poppet may likewise help to reduce leakage but increase the hysteresis of the valve and thus impair performance.
Since the seat stresses in conventional valves generally decrease in constant proportion to the increase in system pressure, such valves generally experience greater leakage at pressures approaching the desired threshold pressure at which the valve opens.